Why don't you try running through the file counting up the number of times a line of a given length is seen... 
my %line_count_by_length;
while (<DATA>) {
    my $line_length = length($_);
    $line_count_by_length{$line_length}++;
}

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If any (or a sufficiently large portion of) those line counts represent a big percentage of the total line count, you could make a guess that the file was fixed width. Perhaps also giving a weighting on how many different line lengths are represented in the file, compared to how many you might expect given the file's length?
---
my name's not Keith, and I'm not reasonable.

In reply to Re: how to identify a fixed width file by reasonablekeith
in thread how to identify a fixed width file by ftumsh

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